1. Field of the Invention
The invention relates to a vibration damper with adjustable damping force of the type having a pressure tube in which a piston rod with a piston is guided with freedom of axial movement, the piston dividing the pressure tube into a working space around the piston rod and a working space away from the piston rod, the working spaces being filled with damping medium.
2. Description of the Related Art
U.S. Pat. No. 6,464,048 discloses a vibration damper with adjustable damping force having a completely closed piston on an axially movable piston rod. As a result, the piston divides the interior of the cylinder into two working spaces, each of which has a fluid connection to an adjustable damping valve, which is controlled in turn by an actuator. The control is implemented via the adjustable throttling of a control stream or sidestream in the damping valve. A common actuator is used for the two adjustable damping valves, one of which is provided for each flow direction. Both the mainstream displaced through each of the damping valves and the sidestream controlled by the actuator are displaced from the working space in question into a common reservoir. Additional supply channels proceed from the reservoir to the two working spaces; the working space which is without pressure at the time in question can thus be supplied with damping medium when the piston rod moves, so that it is impossible for a negative pressure to develop at any time.
A vibration damper having a closed piston is also disclosed in U.S. Pat. No. 4,743,000, where the volume displaced from the compressed working space is displaced in its entirety through the directly controlled damping valve. The description “directly controlled” is to be understood in the sense that the closing force acting on the disk spring of the adjustable damping valve or on the helical valve slide is applied directly by an actuator such as a controlling magnet. In the case of an indirectly controlled damping valve, such as that described in U.S. Pat. No. 6,464,048, use is made of the pressure of a fluid stream, namely, a sidestream, to exert an opening and closing force on a valve body.
The damping medium leaving the damping valve of U.S. Pat. No. 4,743,000 is distributed as a function of volume between the enlarged working space and the connected compensating space. In FIG. 3 of this patent, the compensating space is external to the vibration damper. The cylinder has a fluid connection in each case to a single, directly controlled damping valve, the damping force of which can be adjusted asymmetrically; that is, when a large damping force setting is present in the outward travel direction, a small damping force setting is selected for the inward travel direction and vice versa. In FIG. 5 of this same document, the compensating space is in a ring-shaped area between a container tube and the cylinder. The outlets of the adjustable damping valves are combined into a single fluid connection, which has a branch leading away to the compensating space. The two damping valves according to FIG. 5 are both spatially and structurally separate, and each has its own actuator.
DE 44 23 526 A1 describes a vibration damper with adjustable damping force, in which a damping medium-filled pressure tube is divided by a piston on an axially movable piston rod into a working space on the piston rod side of the piston and a working space on the side away from the piston rod, where a stream of damping medium, which flows between the two working spaces, is divided into a mainstream and a sidestream. In the path traveled by the stream of damping medium, a damping valve device is provided, which consists, for each direction of flow, of a damping valve body with a main stage valve; each main stage valve is formed in turn by a main stage valve body, which defines a control space, the main stage valve body moving axially in the control space in response to the arriving flow. A pilot valve is also provided; the minimum of one main stage valve is controlled by an automatic actuator, which allows a certain sidestream to flow through a connection from the control space to a working space of the pressure tube, and, in both flow directions in each case, the mainstream flows through a main stage valve body of the main stage valve, and the sidestream flows through the pilot valve. So that a “skyhook” automatic control strategy can be realized, the pilot valve has two different control cross sections, which can be used alternately; depending on its direction, the sidestream passes through one of the control cross sections and a nonreturn valve, which opens in the flow direction. The mainstream determines the level of the damping force, whereas the sidestream serves to control the adjustable damping valve with the goal of minimizing the energy input.